The present invention relates to an apparatus for twisting or false twisting multifilament yarns, of the type comprising an endless, flat belt, which is entrained about a number of rollers so that the belt crosses itself at a point with a substantially rectangular cross section to define a twisting zone. A running yarn is guided through the twisting zone and between the belt segments so that a twist is imparted to the yarn.
It is known to guide a running yarn across surfaces which are moved transversely to the yarn path for the purpose of imparting temporary or permanent twist to the yarn, and this procedure permits high yarn speeds and a sufficient level of twist density, since the frictional contact between the surface and the yarn serves to impart a desired twist to the yarn. Known from the art is a type of apparatus in which the yarn is guided between two belts or belt segments which cross each other and move in opposite directions to thereby impart twist to the running yarn and, at the same time, a conveying component is imparted of a magnitude which is dependent on the crossing angle of the belts or belt segments.
Two different forms of apparatus of the described type are known. In one, two endless belts are moved against each other at a crossing angle which is adjustable with the yarn being guided in the crossing area between the two belts. Examples of such apparatus can be found in Swiss Pat. No. 278,535, British Pat. No. 1,083,052, U.S. Pat. No. 4,144,700, and others. However, these prior designs have several disadvantages. For example, it is known that a belt cannot transmit transverse forces without deflection unless auxiliary support means is provided, even under a very high belt tension. For this reason, it will not suffice to simply guide the belts across each other, and the first-mentioned prior art publication thus provides for contact pressure rolls on the backside on both sides of the yarn path, whereas British No. 1,083,052 suggests that one of the two belts be provided with ferrogmagnetic properties and that, in the area of the yarn passage, a magnet be arranged on the backside of the opposite belt. However, neither of these two suggestions has been found to be satisfactory.
In a second form of prior apparatus, only one endless belt is used, which is guided over two rolls so that it crosses between the rolls. This crossing, which is accomplished in that one half of the belt loop is twisted relative to the other by about 180.degree., or even somewhat more if necessary, makes the two belt segments which move against each other in the crossing area contact in such a manner that a twist is imparted to the yarn which is guided between them. U.S. Pat. Nos. 2,908,133, 4,566,265, and British Pat. No. 986,245, disclose apparatus of this type.
This second type of apparatus makes it possible to impart a twist to the yarn. since the belt segments contacting each other in the crossing area, detorm each other. However, the ettectiveness is limited, since the belt crossing angle and thus the amount of twist imparted to the yarn cannot be readily adjusted, and the insertion of the yarn between the crossing belts sections is very burdensome and time consuming.
In addition, only the inner belt surface is used because of the type of belt guidance, and this can lead to changes in the frictional contact between the belt and the yarn, and to uncontrolled drive slippage, by reason of a soiled belt surface resulting from yarn deposits or the like.
Accordingly, it is an object of the present invention to provide an apparatus for twisting a running yarn which overcomes the disadvantages and limitations of the prior art as noted above.
It is also an object of the present invention to provide a twisting apparatus of the type employing a single crossed endless belt, and which provides for the ready adjustment of the belt crossing angle to thereby permit adjustment of the amount of twist imparted to the yarn.
It is a further object to provide an apparatus of the described type, and which has improved contact between the yarn and the belt, which provides for a simple and fast insertion of the yarn, and which may be configured so that both belt sides are used as much as possible.